The development of environmentally-sensitive and cost-effective methods to treat and dispose of acidic waste effluent has taken on considerable importance in recent years.
The success of a waste treatment system depends upon the ability of the system to concentrate the waste effluent sufficiently before it is delivered to the treatment system. This is due to the fact that waste treatment systems are limited by hydraulic load. In addition, it is economically advantageous to use a system which allows the recovery of acidic product of high purity before the waste effluent reaches the treatment system.
An ideal separation process is one which is capable of handling waste effluents with varying concentrations of acid and metal, which can be adapted easily to deal with these varying concentrations in waste effluent solutions and which separates the metal salt from the acid solution with high efficiency.
Pressure-driven systems do not function well if the waste effluent has a high metal concentration. The range of application of pressure-driven membrane separation processes is limited by the osmotic pressure of the metal ions being rejected. Thus, as the metal concentration in the reject stream increases, the pressure which is applied to the membrane must be increased to compensate for the increased osmotic pressure.
Acid-sensitive systems do not operate well if the waste effluent contains a high acid concentration. This is due to the fact that most commercially available separation membranes are sensitive to acid and their life is inversely proportional to the strength of the acid being treated. Thus, if the waste effluent has a high acid concentration, it is necessary to either neutralize some of the free acid in the waste effluent or to remove some of the acid from the waste effluent before it reaches the acid-sensitive membrane.
Both of these means of reducing the acid concentration of a waste effluent have disadvantages, especially in the commercial context. The addition of a neutralization step to the separation process entails additional capital and operating costs for the neutralization chemicals. Acid sorption methods can be used to remove some of the acid from the effluent waste solutions. However, these methods require significant quantities of water to remove the acid from the acid sorption media. This represents an important disadvantage in settings with limited access to sources of water and where the price of water is often quite high.